public class Debug {

    public static void main(String... args) {

        // R(t) = tsin(0.2t)
        Function function = new Function() {
            public double calculate(double input) {
                // R(t) = tsin(0.2t)
                return input * Math.sin(0.2 * input);
            }
        };

        System.out.println(areaUnderCurve(0, 50, 1000000, function));
    }

    /**
     * This method will calculate the area under the curve of the provided function, which in this case, is
     * R(t) = tsin(0.2t), representing by [input * Math.sin(0.2 * input)], input being 't' or 'time.'
     *
     * @param from - the beginning year
     * @param to - the ending year
     * @param rectangles - how many rectangles, or in this case how accurate it is
     * @param function - the function
     *
     * @return the average of the rectangles, calculated from the right-edge and left-edge.
     */
    private static double areaUnderCurve(double from, double to, int rectangles, Function function) {
        double interval = (to - from) / rectangles;

        double left = 0.0;
        for (int i = 0; i < rectangles; i++) { // LEFT
            System.out.println(((double) i / rectangles) * 100 + "% done.");
            left += interval * function.calculate(from + (interval * i));
        }

        System.out.print("\n\nFinished Left! Onto Right!\n\n");

        double right = 0.0;
        for (int i = 1; i <= rectangles; i++) { // RIGHT
            System.out.println(((double) i / rectangles) * 100 + "% done.");
            right += interval * function.calculate(from + (interval * i));
        }

        return (right + left) / 2;
    }
}